C4 2005 Fault code C1302 - A lot of stuff replaced

[lunarband]

Hello. We have been working on a -05 petrol C4 and currently we are running into fault code C1302. The full description in Diagbox is C1302 Incoherence between the brake light switch and the pressure sensor; short circuit, short circuit to ground or open circuit

The fault can be wiped, but reappears on every engine start. Beeps, ABS light lights up and "Braking system faulty" can be seen on the screen. Looking at the live data the brake pressure is like it should be, so are the wheel sensor data. You can drive the car.

What we've done so far:

• Replaced the ABS ECU
• Replaced the ABS pump
• Tried two different brake light switches

to no avail. We are suspecting the wiring between the brake light switch and the ABS unit. How would one go about measuring / identifying any damaged wires there? Are there resources available on the wire color coding?

[wheeler]

I woldnt have thought a 2004 model C4 would have a brake pressure sensor or is this a bad translation?
If you post the VIN i can look out an ABS wiring diagram.

[lunarband]

I woldnt have thought a 2004 model C4 would have a brake pressure sensor or is this a bad translation?
If you post the VIN i can look out an ABS wiring diagram.

This is the VIN we've got. VF7**************[VIN obfuscated, can be read by forum staff]
It says 2007 but the car has been registered since 2005.

[wheeler]

The Brake switch is item 2021

[wheeler]

As i suspected there doesn't seem to be a braking pressure sensor on this car unless it's an integral part of the ESP block?
Normally brake pressure sensors are externally screwed into either the brake master cylinder or directly into the ABS block.

[lunarband]

As i suspected there doesn't seem to be a braking pressure sensor on this car unless it's an integral part of the ESP block?
Normally brake pressure sensors are externally screwed into either the brake master cylinder or directly into the ABS block.

Yes. It's integrated on the hydraulic block as far as I know. Changing the hydraulic block made the idle brake pressure report 0 like it should instead of 4.

Is 2120 what you meant? It's a bit difficult to make out the numbers off that image, but thank you anyway. Any resources on the color coding?

[wheeler]

2120 is the brake position switch. Citroen don’t go by colours, the go by codes printed on the wires. You can see them on the diagram but you may need to zoom in.

[lunarband]

2120 is the brake position switch. Citroen don’t go by colours, the go by codes printed on the wires. You can see them on the diagram but you may need to zoom in.

Can't find item 2021 on the diagram.

[wheeler]

Its 2120 not 2021.
I have edited the diagram to highlight it.

[lunarband]

Its 2120 not 2021.
I have edited the diagram to highlight it.

Yeah, found that already. Thanks! Got confused by your previous post.

The Brake switch is item 2021

So, should the wires have a 2- or 4-digit number marked on them?

[wheeler]

Wire codes are normally anything from 2 - 6 digits & can contain letters and numbers
Pin 1 of the brake switch is BE10A and pin 2 is 210B.
The bit that says 4V VE means its a 4 way green connector.

[GiveMeABreak]

Ok, as per your PM:

Fault Code: C1302	Braking pressure sensor signal fault: Open circuit, short circuit to positive, short circuit to earth
Description of Fault:	The supply voltage of the pressure sensor inside the ESP ECU is monitored every 40 milliseconds. Possibility 1 The output voltage from the pressure sensor is outside the authorised range. Possibility 2 The supply voltage of the pressure tester is not within the nominal range.
Conditions for Fault to clear:	Possibility 1 The output voltage from the pressure sensor inside the ESP ECU is within the authorised range. Possibility 2 The supply voltage of the pressure sensor is within the nominal range.
Downgrade Modes whilst Fault is active:	The ABS operation that is current goes its term. Once it is finished, cutting of all ABS operations. Immediate cutting of all the speed control functions (Electronic stability control (ESP)) The electronic brakeforce distribution function is in down-grade mode
Symptoms:	Lighting of the ABS and ESP warning lamps
Suspect Areas:	Pressure sensor inside the ESP ECU Presence of air bubbles in the ESP hydraulic block Brake light switch Connectors Electrical harnesses Fuses

Wiring Diagrams

[Carlos Fandango]

Do you ever find out what the problem was? Got the same code but passes the pressure check so the ABS pump and ECU appear to be working ok.
Many thanks

[GiveMeABreak]

The other suspect areas are listed in the fault code table.

If you've undertaken jobs like replacing brake calipers, or any other operation that requires changing the brake fluid, has the system been bled correctly (twin operations - both manually and using the diagnostic tool for the hydraulic block in the ESP)

If not applicable, then work through the other suspects, like the wiring harness, plug connections and pins as these can become loose.

Also the brake light switch is another component, so replacing that should be considered.

[Carlos Fandango]

Anyone reading this post with the same fault, this is what I did to fix the problem.
2008 2.0HDI
Used Diagbox to fault find but everything seemed normal. Brake switch was changing state when the pedal pushed down and brake pressure went from 0 to 70bar and back down to 0.
Started looking through the circuit diagrams which confused things even more.
Only thing we did notice was when pressing the brake pedal and switching on the ignition, the ABS/ESP lights remained off until the pedal was released. This was the same after starting the engine, release the pedal the lights come on.
After some good advice it was suggested that it could either be the brake switch or the ABS pump.
As we had done 2 switches plus a brand new one we went for the pump.
The pump on this car also has attached to it the ESP ECU and in this combination is the pressure sensor.
After ordering and fitting an exact part number match from eBay we started off with a convention bleed which got us no where. Pressure bleeder fitted which started getting the majority of the air out the system but the pedal operation was still poor. Diagbox connected again and a full ABS bleed carried out. Loads more air out with now normal pedal operation but most important, ABS/ESP lights are gone